In electronic circuits such as semiconductor driver circuits, there exists the possibility that the output can be shorted to a circuit reference potential, such as ground. It is also possible for the output to be shorted to the circuit supply potential hereinafter referred to as V.sub.DD. By a short circuit condition it is meant that the output becomes connected, through a very low impedance path, to either ground or circuit supply, V.sub.DD, irrespective of the input signal.
When uncontrolled, short circuit condition can result in component burnout due to excessive current flow over the short circuit path between the component and the shorting potential, be it ground or supply potential. As explained in U.S. Pat. No. 3,749,936 to Bell, which describes an output protection circuit of the prior art, semiconductor devices have finite breakdown voltages and current limitations. As such, while a semiconductor device can withstand short periods of high current or longer periods of lower current, if these limitations are exceeded the device will be over stressed and may become permanently inoperable.
While semiconductor circuits are designed to control current to the semiconductor device to within safe limits, circuit malfunctions may result in short circuiting the output with attendant long periods of high current. To protect against this occurrence it is known to provide semiconductor circuits with added fault protection circuitry. The Bell patent describes such a fault protection circuit in the form of a clock signal dependent, feedback loop.
The Bell feedback loop functions to prevent burnout of either of the circuit's output transistors, the circuit output being taken at a point between these two transistors. One output transistor connects the circuit supply to the output node, while the second transistor connects the output node to circuit ground. Under normal operation, only one of these two output transistors would be conductive at any moment. When the output is signaled to assume ground potential, the transistor between the supply source and the output node is off while the transistor between the output and ground is rendered conductive. On the other hand, when the output is signaled to assume the potential of the supply, the transistor between the output and ground is turned off and the transistor between the supply, V.sub.DD, and the output terminal is turned on.
Considering the first case mentioned, when the output is signaled to assume ground potential, if the output becomes shorted to supply potential, V.sub.DD, in the presence of a signal ordering the output to assume ground potential thus turning on the transistor between the output and ground, a potential drop appears across this conducting transistor equal to the full value of the potential difference between supply and ground. High current flows, being much greater than that for which the circuit was designed and component burnout is likely.
In the latter mentioned case, in the presence of an output shorted to ground, the conducting transistor between the output and the supply bears the entire supply to ground potential drop and excessive current once again flows.
The Bell protection circuit operates to command the input to assume that state required to produce an output state corresponding to the shorted condition. For example, if the output were shorted to ground, the feedback loop, in response to clock signals, commands the input to assume a state which in normal circuit operation would cause the output to assume the ground state. The short to ground then becomes a harmless fault since this circuit is in the output to ground state and the components are in such states as to be compatible with a grounded output condition. Therefore, component damage is prevented.
In an identical manner, with the output shorted to supply potential the feedbck loop, in response to clock signals, commands the input to assume a state which forces the output to assume V.sub.DD. Thus, the short to supply becomes a harmless fault since the circuit is in the output to supply state and the components are in such state as to be compatible with an output at supply potential.
The Bell device is clock signal dependent. That is, it will only work if the clock is running. Since it is clock signal dependent, the circuit response to alleviate a short is necessarily slow. This is so for when a short is removed the output will remain in the opposite state until the next clock pulse. A short circuit protection apparatus which is clock independent would indeed be advantageous.